1. Field of the Invention
This invention relates generally to a method for embedding a hydrophilic and electrically conductive layer into a bipolar plate for a fuel cell and, more particularly, to a method for embedding a niobium doped titanium oxide powder into an outer surface of a bipolar plate for a fuel cell to make the plate electrically conducted and hydrophilic.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are popular fuel cells for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane defines a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The automotive fuel cell stack mentioned above may include two hundred or more fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The bipolar plates are typically made of a conductive material, such as stainless steel, titanium, aluminum, polymeric carbon composites, etc., so that they conduct the electricity generated by the fuel cells from one cell to the next cell and out of the stack. Metal bipolar plates typically produce a natural oxide on their outer surface that makes them resistant to corrosion. However, the oxide layer is not conductive, and thus increases the internal resistance of the fuel cell, reducing its electrical performance. Also, the oxide layer makes the plate less hydrophilic.
As is well understood in the art, the membrane within a fuel cell needs to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. During operation of the fuel cell, moisture from the MEAs and external humidification may enter the anode and cathode flow channels. At low cell power demands, typically below 0.2 A/cm2, water may accumulate within the flow channels because the flow rate of the reactant gas is too low to force the water out of the channels. As the water accumulates, it forms droplets that continue to expand because of the relatively hydrophobic nature of the plate material. The contact angle of the water droplets is generally about 90° in that the droplets form in the flow channels substantially perpendicular to the flow of the reactant gas. As the size of the droplets increases, the flow channel is closed off, and the reactant gas is diverted to other flow channels because the channels are in parallel between common inlet and outlet manifolds. Because the reactant gas may not flow through a channel that is blocked with water, the reactant gas cannot force the water out of the channel. Those areas of the membrane that do not receive reactant gas as a result of the channel being blocked will not generate electricity, thus resulting in a non-homogenous current distribution and reducing the overall efficiency of the fuel cell. As more and more flow channels are blocked by water, the electricity produced by the fuel cell decreases, where a cell voltage potential less than 200 mV is considered a cell failure. Because the fuel cells are electrically coupled in series, if one of the fuel cells stops performing, the entire fuel cell stack may stop performing.
It is usually possible to purge the accumulated water in the flow channels by periodically forcing the reactant gas through the flow channels at a higher flow rate. However, on the cathode side, this increases the parasitic power applied to the air compressor, thereby reducing overall system efficiency. Moreover, there are many reasons not to use the hydrogen fuel as a purge gas, including reduced economy, reduced system efficiency and increased system complexity for treating elevated concentrations of hydrogen in the exhaust gas stream.
Reducing accumulated water in the channels can also be accomplished by reducing inlet humidification. However, it is desirable to provide some relative humidity in the anode and cathode reactant gases so that the membrane in the fuel cells remains hydrated. A dry inlet gas has a drying effect on the membrane that could increase the cell's ionic resistance, and limit the membrane's long-term durability.
It has been proposed in the art to make bipolar plates for a fuel cell hydrophilic to improve channel water transport. A hydrophilic plate causes water in the channels to form a thin film that has less of a tendency to alter the flow distribution along the array of channels connected to the common inlet and outlet headers. If the plate material is sufficiently wettable, water transport through the diffusion media will contact the channel walls and then, by capillary force, be transported into the bottom corners of the channel along its length. The physical requirements to support spontaneous wetting in the corners of a flow channel are described by the Concus-Finn condition,
            β      +              α        2              <          90      ⁢      °        ,where β is the static contact angle and α is the channel corner angle. For a rectangular channel α/2=45°, which dictates that spontaneous wetting will occur when the static contact angle is less than 45°. For the roughly rectangular channels used in current fuel cell stack designs with composite bipolar plates, this sets an approximate upper limit on the contact angle needed to realize the beneficial effects of hydrophilic plate surfaces on channel water transport and low load stability.
The various surface layers proposed in the art to make the bipolar plate hydrophilic also need to have a low contact resistance so that the plate is sufficiently electrically conductive. U.S. patent application Ser. No. 11/172,021, filed Jun. 30, 2005, titled Stable Conductive and Hydrophilic Fuel Cell Contact Element, assigned to the Assignee of this application and herein incorporated by reference, discloses various hydrophilic and electrically conductive layers. Specific materials include niobium (Nb) and tantalum (Ta) doped titanium oxide (TiO2), tantalum doped (T) TiO2 and fluorine (F) doped tin oxide (SnO2).